Group membership rule errors

New to Okta, trying out group membership rules. According to https://help.okta.com/en/prod/Content/Topics/Directory/Directory_Groups.htm#ConditionsEE, “Any function supported in Okta Expression Language.” is supported. But when I enter this as a rule (just as an experiment):

Convert.toInt('1') > 0

I get an error:

Unsupported expression constructs are used in expression Convert.toInt('1') > 0

What am I missing? Thanks!